This invention relates generally to magnetic recording media, and more particularly to thermally stable high density media.
Conventional magnetic recording media, such as the magnetic recording disks in hard disk drives, typically use a granular ferromagnetic layer, such as a sputter-deposited cobalt-platinum (CoPt) alloy, as the recording medium. Each magnetized domain in the magnetic layer is comprised of many small magnetic grains. The transitions between magnetized domains represent the xe2x80x9cbitsxe2x80x9d of the recorded data. IBM""s U.S. Pat. Nos. 4,789,598 and 5,523,173 describe this type of conventional rigid disk.
As the storage density of magnetic recording disks has increased, the product of the remanent magnetization Mr (the magnetic moment per unit volume of ferromagnetic material) and the magnetic layer thickness t has decreased. Similarly, the coercive field or coercivity (Hc) of the magnetic layer has increased. This has led to a decrease in the ratio Mrt/Hc. To achieve the reduction in Mrt, the thickness t of the magnetic layer can be reduced, but only to a limit because the layer will exhibit increasing magnetic decay, which has been attributed to thermal activation of small magnetic grains (the superparamagnetic effect). The thermal stability of a magnetic grain is to a large extent determined by KuV, where Ku is the magnetic anisotropy constant of the layer and V is the volume of the magnetic grain. As the layer thickness is decreased, V decreases. If the layer thickness is too thin, the stored magnetic information will no longer be stable at normal disk drive operating conditions.
One approach to the solution of this problem is to move to a higher anisotropy material (higher Ku). However, the increase in Ku is limited by the point where the coercivity Hc, which is approximately equal to Ku/Mr, becomes too great to be written by a conventional recording head. A similar approach is to reduce the Mr of the magnetic layer for a fixed layer thickness, but this is also limited by the coercivity that can be written. Another solution is to increase the intergranular exchange, so that the effective magnetic volume V of the magnetic grains is increased. However, this approach has been shown to be deleterious to the intrinsic signal-to-noise ratio (SNR) of the magnetic layer.
It is known that substantially improved SNR can be achieved by the use of a laminated magnetic layer of two (or more) separate magnetic layers that are spaced apart by an nonmagnetic spacer layer. This discovery was made by S. E. Lambert, et al., xe2x80x9cReduction of Media Noise in Thin Film Metal Media by Laminationxe2x80x9d, IEEE Transactions on Magnetics, Vol. 26, No. 5, September 1990, pp. 2706-2709, and subsequently patented in IBM""s U.S. Pat. No. 5,051,288. The reduction in intrinsic media noise by lamination is believed due to a decoupling of the magnetic interaction or exchange coupling between the magnetic layers in the laminate. The use of lamination for noise reduction has been extensively studied to find the favorable spacer layer materials, including Cr, CrV, Mo and Ru, and spacer layer thicknesses, from 5 to 400 xc3x85, that result in the best decoupling of the magnetic layers, and thus the lowest media noise. This work has been reported in papers by E. S. Murdock, et al., xe2x80x9cNoise Properties of Multilayered Co-Alloy Magnetic Recording Mediaxe2x80x9d, IEEE Transactions on Magnetics, Vol. 26, No. 5, September 1990, pp. 2700-2705; A. Murayama, et al., xe2x80x9cInterlayer Exchange Coupling in Co/Cr/Co Double-Layered Recording Films Studied by Spin-Wave Brillouin Scatteringxe2x80x9d, IEEE Transactions on Magnetics, Vol. 27, No. 6, November 1991, pp. 5064-5066; and S. E. Lambert, et al., xe2x80x9cLaminated Media Noise for High Density Recordingxe2x80x9d, IEEE Transactions on Magnetics, Vol. 29, No. 1, January 1993, pp. 223-229. U.S. Pat. No. 5,462,796 and the related paper by E. Teng et al., xe2x80x9cFlash Chromium Interlayer for High Performance Disks with Superior Noise and Coercivity Squarenessxe2x80x9d, IEEE Transactions on Magnetics, Vol. 29, No. 6, November 1993, pp. 3679-3681, describe a laminated low-noise disk that uses a discontinuous Cr film that is thick enough to reduce the exchange coupling between the two magnetic layers in the laminate but is so thin that the two magnetic layers are not physically separated.
What is needed is magnetic recording media that will support very high density recording while retaining good thermal stability and SNR.
The invention is a magnetic recording medium wherein the magnetic recording layer is at least two ferromagnetic films antiferromagnetically coupled together across a nonferromagnetic spacer film. Because the magnetic moments of the two antiferromagnetically-coupled films are oriented antiparallel, the net remanent magnetization-thickness product (Mrt) of the recording layer is the difference in the Mrt values of the two ferromagnetic films. This reduction in Mrt is accomplished without a reduction in the thermal stability of the recording medium because the volumes of the grains in the antiferromagnetically-coupled films add constructively. The medium also enables much sharper magnetic transitions to be achieved with reduced demagnetization fields, resulting in a higher linear bit density for the medium. In one embodiment the magnetic recording medium comprises two ferromagnetic films, each a granular film of a sputter deposited CoPtCrB alloy, separated by a Ru spacer film having a thickness to maximize the antiferromagnetic exchange coupling between the two CoPtCrB films. One of the ferromagnetic films is made thicker than the other, but the thicknesses are chosen so that the net moment in zero applied magnetic field is low, but nonzero.